One of his most notable roles was that of Aragorn in The Lord of the Rings trilogy, which elicited an overwhelming fan mail volume for this shy loner. He got a call one day asking him to replace the Irish actor Stuart Townsend, who, Peter Jackson had decided, looked too young to be Aragorn. Viggo was asked to board a plane the next day to go to New Zealand. He hadn't read J. R. R. Tolkien's The Lord of the Rings before, and he wouldn't have gone if it hadn't been for his son Henry, who had read the books and was extremely enthusiastic about them.
After several years of experience in live theater, he began his movie career as an Amish farmer in Peter Weir's Witness. In spite of his striking looks, he is a highly changeable character actor, who has appeared in Jane Campion's film of Henry James's Portrait of a Lady, Sean Penn's Indian Runner, Brian DePalma's Carlito's Way, Ridley Scott's G.I. Jane and Tony Goldwyn's A Walk on the Moon.
Mortensen's performance in Bent at the Coast Playhouse, Los Angeles, won him a Dramalogue Critics' Award.
Mortensen has published two volumes of poetry. His marriage in 1987 to punk rocker Exene (Christine) Cervenka, ended in an amicable divorce in 1997. They have a son, Henry.
Mortensen spent some childhood years in Venezuela, Argentina, and his father's native Denmark, before returning to high school in Watertown, New York (where he was captain of the swim team). He graduated from St. Lawrence University in 1980. He is fluent in English, Spanish, Danish, and several other languages.